I have the RX Vega 64 and a FreeSync supporting monitor(Samsung C24FG70) Whenever FreeSync is on and the games FPS is in the FreeSync range of 70-144 I get a noticeable amount of flickering which makes the experience feel jittery and causes headaches. This is especially severe in Destiny 2 where there is a large amount of brightness flickering making it a significantly worse experience. Any help is appreciated!
Sounds like this issue is pretty prominent with those monitors. It is also observed with NVidia GPUs tested with their new adaptive sync driver.
Scroll to the end of that discussion, and look at the monitor list. Not much you can do about it other that try to get updated firmware from Samsung.